Evolutionary improvement of batteries is the most important element in recent advancements of electric automobiles and mobile electrical devices. A battery separator is a basic structural element of a battery, and improving performance of the separator is an important factor in achieving higher battery performance and making batteries smaller and safer. A porous film comprising a polyolefin film has previously been used as a battery separator, but the film method had a limitation of increasing the number of pores. Therefore, attempts have been made to create a non-woven cloth separator using microfibers (JP2004-15980A and the like). However, the use of a sea-islands structure type fiber method to manufacture a fine filament made the process complicated. The filament obtained in this manner was not sufficiently fine, and the steps needed to cut the filament and convert the short fibers into a non-woven cloth were complex, creating problems in terms of both quality and cost.
The electro-spinning method (henceforth referred to as the ES method) is a typical technique used to create fibers used in non-woven cloth as fine in a nanoscale as possible (You, Y., et. al “Journal of Applied Polymer Science, Vol. 95, p. 193-200, 2005”). However, the ES method, which involves dissolving a polymer in a solvent and applying high voltage, is unacceptable in the case that there is no safe solvent, such as in the case of polyolefin. In addition, a web created using the ES method does not contain continuous filaments and also contains short fibers since the filaments are cut during spinning. One of the problems is the generation of many molten resin lumps that are larger than the fiber diameter due to the shrinkage of cut sections during spinning. In addition, a molten type ES method that was conducted without using a solvent was tried (JP2009-299212A and the like), but the average fiber diameter did not reach a nanoscale. In addition, many defects greater than the fiber diameter were thought to be generated, resulting in problems similar to those observed in the solvent method. Furthermore, both a laser and a high voltage electrode were used in the manufacturing process which results in a complex process and causes both quality and cost problems when the process is used to manufacture a battery separator. Moreover, a melt-blown method is also one of the manufacturing processes used to create a super fine filament non-woven cloth, but the filament diameter reaches a nanoscale only with difficulty and the generation of molten resin lumps causes similar problems.
The present invention is obtained applying the technology which was a filament drawing technology using infrared radiation for heating under reduced pressure and an application of which was previously filed by the inventors (International Publication Number WO2008/084797A1). The present invention further improves the technology so that the technology can be effectively applied to a battery separator comprising a porous polyolefin sheet.